A group of Democrat senators have written a letter probing the resignation of White House Staff Secretary Rob Porter amid allegations of domestic abuse.

In a letter addressed to White House Chief of Staff Gen. John Kelly and White House Counsel Don McGahn, 12 Democratic senators led by Sen. Richard Blumenthal (D-CT) ask whether Kelly was previously made aware of the allegations against him. The letter also asks why Porter did not have security clearance.

“We recognize that you and the President have tremendous discretion in deciding whom to hire to work in the White House,” the letter reads.

“However, we are troubled by published accounts suggesting that you decided to hire Mr. Porter despite the fact that he could not get a security clearance and that you were aware of the specific domestic violence allegations made against him,” it continues.

In addition to Porter, why are “multiple aides” who have also been denied a security clearance allowed continued access to our nation’s most sensitive secrets? I wrote to the White House with eleven of my colleagues demanding an explanation pic.twitter.com/zoVcV3vkxG

The letter follows reports suggesting that both Kelly and McGahn had been made aware of the allegations against Porter but failed to act on them.

According to a report from ABC News, corroborated by sources close to Breitbart News, Kelly has expressed his willingness to resign over the matter, although he has since denied this.

Porter, who is currently in a relationship with White House communications director Hope Hicks, announced his resignation last Wednesday after the unearthing of domestic abuse allegations from two of his ex-wives, Colbie Holderness and Jennifer Willoughby, who shared stories and photo evidence of alleged repeated physical abuse.

However, Porter has denied all the allegations against him, describing them as a “coordinated smear campaign.”